Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPP third generation partnership project
UTRAN universal terrestrial radio access network
Node B base station
UE user equipment
EUTRAN evolved UTRAN (also referred to as LTE)
LTE long term evolution
aGW access gateway
eNB EUTRAN Node B (evolved Node B)
MAC medium access control
MM mobility management
PDU protocol data unit
PRB physical resource block
PHY physical
RLC radio link control
RRC radio resource control
RRM radio resource management
PDCP packet data convergence protocol
HARQ hybrid automatic repeat request
C-RNTI cell radio network temporary identifier
RA-RNTI random access radio network temporary identifier
ACK acknowledge
NACK not acknowledge
OFDMA orthogonal frequency division multiple access
SC-FDMA single carrier, frequency division multiple access
UL uplink
DL downlink
FSP frequency shift peak
RACH random access channel
zC Zadoff-Chu
IDFT inverse discreet Fourier transform
PDSCH physical downlink shared channel
PDCCH physical downlink control channel
PRACH physical random access channel
A proposed communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or as E-UTRA) is currently under development within the 3GPP. The current working assumption is that the DL access technique will be OFDMA, and the UL access technique will be SC-FDMA.
One specification of interest to these and other issues related to the invention is 3GPP TS 36.300, V8.3.0 (2007-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8).
FIG. 1A reproduces FIG. 4 of 3GPP TS 36.300, and shows the overall architecture of the E-UTRAN system. The E-UTRAN system includes eNBs, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an EPC, more specifically to a MME (Mobility Management Entity) by means of a S1-MME interface and to a Serving Gateway (S-GW) by means of a S1-U interface. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs.
The eNB hosts the following functions:
functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);
IP header compression and encryption of user data stream;
selection of a MME at UE attachment;
routing of User Plane data towards Serving Gateway;
scheduling and transmission of paging messages (originated from the MME);
scheduling and transmission of broadcast information (originated from the MME or O&M); and
measurement and measurement reporting configuration for mobility and scheduling.
Reference can also be made to 3GPP TS 36.321, V8.0.0 (2007-12), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) Medium Access Control (MAC) protocol specification (Release 8).
Of particular interest herein is the random access procedure of the LTE (E-UTRA) system. The procedure is described in 3GPP 36.300 v.8.3.0, and its steps are shown in FIG. 1B, which reproduces Figure 10.1.5.1-1: Connection based Random Access Procedure, of 3GPP TS 36.300. The steps shown in FIG. 1B are described in detail in subclause 10.1.5.1 of 3GPP TS 36.300.
Briefly, the UE transmits a random access preamble and expects a response from the eNB in the form of a so-called Message 2. Message 2 is transmitted on the PDSCH and its resources are allocated on the PDCCH as for any DL message. The resource allocation of Message 2 is addressed with an identity RA-RNTI that is associated with the frequency and time resources of a PRACH, but is common for the different preamble sequences. The Message 2 contains UL allocations for the transmissions of a Message 3 in the UL (step 3 of the random access procedure).
It should thus be appreciated that the LTE system as currently proposed will require a RACH preamble detection algorithm at the eNB. Related to RACH preamble detection, a LTE cellular network that supports high-speed UEs will require careful planning, since the 3GPP specifications require supporting mobile speeds at least up to 350 km/h. It these speeds Doppler induced frequency offsets between the UE and the eNB can be significant.
Zadoff-Chu (ZC) sequences have been selected as preamble sequences for LTE. One reason for this is that one primary (mother) ZC sequence, due to its ideal cyclic auto-correlation properties, can be used to generate several preamble sequences. However, ZC sequences are known to be sensitive to frequency offsets induced by, for example, Doppler shift. Detecting received preambles at the eNB becomes more challenging as the frequency offset increases. This is due at least in part to the fact that additional correlation peaks appear in the time domain at the output of the receiver correlation function. From a system performance point of view this can lead to increased numbers of false alarm events, e.g., network resources are scheduled by the eNB to non-existent UEs due to the false detection of UL RACH signals.
The degradation of the auto-correlation properties leads to the appearance of FSPs in the correlation output. From the receiver point of view the energy of a preamble sequence transmitted by a given UE is dispersed amongst a main correlation peak and several FSPs. When the frequency offset exceeds about 625 Hz the energy contained in the strongest FSP can actually exceed the energy of the main correlation peak. The FSPs can be particularly problematic since the cyclic shifted versions of one ZC sequence are used as different preambles. As a result one or more FSPs may be incorrectly identified as being actual correlation peaks of another preamble sequence generated from the same mother ZC sequence. Hence, the numbers of false alarm events tend to increase as the auto-correlation properties degrade. This problem is particularly troublesome when the SNR at the receiver is high, since FSPs are more exposed behind the noise floor and, therefore, an incorrect decision concerning the presence of a preamble in the received signal becomes more probable.